Industrial material handling vehicles such as fork lift trucks or motorized hand pallet trucks are commonly found in warehouses, factories, and, generally, wherever pallets, packages, or loads of goods are required to be moved from place to place. Pallet trucks typically include a load bearing fork or lift arm for lifting packages or pallets to a height sufficient for transporting, an electric drive motor for driving the vehicles, a steering control mechanism, and a brake. These vehicles can include an operator station, on which the operator stands as the pallet truck moves, or can be designed for the operator to walk behind the vehicle at the end opposite the forks.
The steering mechanism for a common type of pallet truck includes a movable arm or tiller and a control handle mounted at the end of the tiller. The tiller is rotatable right and left to steer the vehicle, while a rotatable thumb wheel or twist grips on the handle control the speed and direction of the truck, selecting between a forward and a reverse direction. To prevent movement of the truck when the operator has left the vehicle, the steering tiller arm is typically spring loaded. When the tiller is released, it is forced by the spring to a near vertical position outside of a defined operating arc. In the vertical position, a spring-applied “deadman” brake mechanism is automatically activated to prevent further motion of the vehicle.
In another type of material handling vehicle, the tiller arm is used to establish a speed range for the vehicle as the tiller arm is rotated between the vertical and horizontal position. In these vehicles, a slower mode of operation is typically provided when the tiller arm is in the near vertical position, and a faster mode or modes of operation are allowed as the tiller is moved toward the horizontal. The driver changes the mode by changing the angle of the tiller arm. These vehicles are particularly useful, for example, in narrow lanes and, in an increasingly common mode of operation, inside of trailers and other large containers for moving goods, as the vehicle can be controlled at a slow speed with the tiller in a nearly vertical position. These vehicles, however, suffer from certain disadvantages. When the truck transitions from slow to fast, typically a rapid increase in travel speed occurs. This rapid acceleration of travel speed is desirable when the operator is prepared to walk at a faster rate, but not (for example) when the operator is maneuvering the truck in a tight area and inadvertently crosses the threshold. This invention creates a smooth transition to fast mode in the event the operator has been operating in slow mode, but also allows for the desirable acceleration to fast mode if the slow mode zone is quickly passed through.